The present invention relates to a method for imparting hydrophilicity to a substrate whereby high hydrophilic properties and water-holding properties can be maintained for a long period of time.
Various measures such as the use of defogging mirrors or eyeglasses have been taken to prevent fog due to vapor or rain. For example, there is a method for applying a surface active agent to the surface of a glass or lens (hereinafter referred to as xe2x80x9csubstratexe2x80x9d), a method for applying a water absorbing agent to the surface of the substrate, a method for applying a water repellent agent to the surface of the substrate, or a method for coating the surface of the substrate with inorganic hydrophilic substance by a wet method.
In the method for applying the surface active agent to the surface of the substrate, by positioning a hydrophilic group on the surface of the substrate, a water screen is uniformly leveled without water adhering to produce a defogging effect.
The method for applying the water absorbing agent to the surface of the substrate is to produce a defogging effect by causing water adhering to the surface of the material to be absorbed by the water absorbing agent.
In the method for applying the water repellent agent to the surface of the substrate, by providing a coating such as a fluoroethylene resin whose surface tension is less on the substrate to increase the contact angle of water droplets, water adhering to the surface is repelled to produce a defogging effect.
In the method for coating the surface of the substrate with an inorganic hydrophilic substance, by providing a hydrophilic thin film with a small contact angle for the water droplets on the surface of the substrate, the water screen is made uniformly level without water adhering to produce a defogging effect, wherein a wet method such as a sol-gel method or a spin coat method is adopted as a coating process.
In the method for applying the surface active agent, there is a problem that the surface active agent cannot be used for a long time because it is easily swept away by water.
In the method for applying the water absorbing agent, there is a problem that the water absorbing agent cannot withstand long term use because it will soon be saturated if there is a lot of water or if an extremely thick water absorbing agent coating is required to allow the water absorbing agent to exhibit a water absorbing effect to some extent and as a result, the reflected image is distorted, or the water absorbing agent exhibits inferior flaw resistance because of the presence of an organic material.
In the method for applying the water repellent agent, there is a problem that though the water repellent agent repels water, it does not have sufficient defogging properties to repel even minute water droplet and it is difficult to maintain the defogging property for a long time.
Further, in the method for coating with an inorganic hydrophilic substance, there is a problem that uniform coating is not possible over a large area such as a mirror because it is difficult to control the coating thickness in the order of nm. It is complicated because there is a plurality of treating processes such as spraying of coating solution and burning, and it is difficult to maintain the defogging properties for a long time because the mechanical strength on the coating surface is insufficient.
To solve the above-mentioned problems, according to the present invention, an SiO2 film is formed directly or through an undercoat layer on a substrate under a reduced pressure of 100 Pa or less and immediately after the SiO2 film is formed, the SiO2 film is treated with water to selectively form silanol groups (SiOH) on the surface of the SiO2 film. The silanol groups (SiOH) formed on the SiO2 film exhibit hydrophilic properties.
In the present invention, the term xe2x80x9cSiO2 filmxe2x80x9d is used to clearly express a silicon dioxide film, but this also includes a film in which the ratio of an Si atom to an O atom is not always 1:2 stoichiometrically. This also applies to an SnO2 film.
A TiO2 film, an Al2O3 film, a Nb2O5 film, a laminated film prepared by laminating the TiO2 film on the Nb2O5 film, or a low emissivity film is desirable as the undercoat layer. In this case, it is desirable that another SiO2 film be formed between the undercoat layer and the substrate. It is also desirable that the SnO2 film be selected as a structure for the low emissivity film. If the SnO2 film which is formed through a laminated film prepared by laminating the SnO2 film and the SiO2 film on the substrate in turn by a CVD method is selected, it is possible not only to decrease an interference reflected color of a thin film, but also to prevent seepage of alkalis (Na) from a glass substrate.
An SiN film is also effective as the undercoat layer provided to prevent seepage of the alkalis (Na). A thickness of 1xcx9c20 nm is desirable for the SiN film.
As described above, the SiO2 film is formed under a reduced pressure of 100 Pa or less. However, there is a method for forming the SiO2 film by a plasma CVD under an atmosphere of 100xcx9c1 Pa of which the main components are silane gas and oxidizing gas, a method for forming the SiO2 film by sputtering under a reduced pressure of 10xcx9c0.1 Pa, or a method for forming the SiO2 film by vacuum deposition under a reduced pressure of 1 Pa or less, or the like. Above all, the method for forming the SiO2 film by sputtering is most excellent as a method for treating a larger area. When the SiO2 film is formed by sputtering an Si target under an oxygen atmosphere, if 1xcx9c400% (preferably 1xcx9c200%, more preferably 60xcx9c80%) of argon or nitrogen is mixed relative to the oxygen in an oxygen atmospheric gas, it is possible to selectively form a silanol group by providing a large oxygen deficiency on the surface of the SiO2 film.
In the case where a glass substrate is selected for the substrate, it is also possible to obtain a hydrophilic treating surface by burning, after the water treatment to form silanol groups, the glass substrate at a temperature of 300xcx9c500xc2x0 C. under an atmospheric pressure in which 0xcx9c400% of nitrogen is provided relative to air. In this manner, it is possible to remove any organic substance adhering to the surface which would become water repellent component and to form a highly hydrophilic film.
It is also desirable that 0.1xcx9c20% by weight of Al component be used to dope an Si component in the SiO2 film. In this manner, adsorption of water on the silanol group is stabilized and a film with high water-holding properties can be formed.
Further, it is desirable that the substrate have transparency or mirror reflection and the transparency or the mirror reflection be maintained even after the hydrophilic treatment.
Preferred embodiments of the present invention will now be explained hereunder.
In a method for imparting hydrophilic properties to a substrate according to the present invention, an SiO2 film is formed on the substrate by a film-forming method under a reduced pressure and immediately after formation thereof, the SiO2 film is treated with water. There is no specific limit to the material for the substrate used in the present invention as far as required to provide the substrate with a defogging function, hydrophilic function or water-holding function. For example, a suitable material includes glass, a mirror, a lens (including a plastic lens), tile or an aluminum panel.
As methods for forming an SiO2 film on the substrate under a reduced pressure of 100 Pa or less, there are a plasma CVD method, a sputtering method, a vacuum deposition method and the like. The reason why a silanol group is formed by performing water treatment immediately after an SiO2 film is formed under a highly reduced pressure is because water reacts before chemical adsorption of an organic substance onto the surface of chemically unstable SiO2 film formed by a film forming method under reduced pressure, oxidation at a surface-active site, or connection of an organic substance at the active site is generated and therefore the silanol group which contributes to the hydrophilic properties is easily formed.
In particular, since the method for sputtering Si as a target in an oxygen atmosphere under a highly reduced pressure can form many active sites on the surface of the substrate, if this is combined with the water treatment immediately after the formation of the film, it is possible to form a large number of silanol groups (SiOH) with highly hydrophilic properties on the surface of the SiO2 film.
By forming a large number of silanol groups on the surface of the SiO2 film, the treated material can exhibit highly hydrophilic properties and water-holding properties. Since the Si of the silanol group when formed by the sputtering method is firmly coated on the surface of the substrate, it is possible to maintain the hydrophilic properties and water-holding properties for a long time.
Non-deionized water may be used in the water treatment. However, it is desirable that distilled water, pure water, or acid solution adjusted by adding an inorganic acid such as hydrochloric acid, sulfuric acid, nitric acid or phosphoric acid, or an organic acid such as acetic acid, butyric acid or lactic acid to the pure water, or an alkaline solution adjusted by adding a base such as sodium hydroxide, potassium hydroxide or ammonium hydroxide to the pure water be used. As a method for water treatment, there is a method for dipping a material formed with an SiO2 film in water or a method for passing the material through water vapor, a method for spraying on the material, or the like.
As an atmospheric gas in the case where the SiO2 film is formed, silane gas of 100xcx9c1 Pa and an oxidizing gas or the oxidizing gas mixed with rare gas or hydrogen gas are used in the plasma CVD. In the vacuum deposition method, there is a method for depositing an SiO2 source by reducing the atmospheric pressure to 0.1 Pa or less, or a method for utilizing an SiOx (X=0xcx9c2) source under an atmosphere of gaseous oxygen of 1xcx9c0.1 Pa or the rare gas or mixed gas thereof.
An atmospheric gas used in the sputtering method may be gaseous oxygen alone, but a method for mixing 1xcx9c400% by volume, preferably 1xcx9c200% by volume, more preferably 60xcx9c80% by volume of argon gas and/or nitrogen gas relative to oxygen may be used. When the mixed gas is used, an SiOx suboxide film is formed and oxygen deficiency, i.e. a large number of active sites exist on the surface of the SiOx suboxide film. As a result, it is possible to form a large number of silanol groups.
Further, as an other means for forming a large number of silanol groups, before forming the SiO2 film, it is also desirable that an undercoat layer consisting of a TiO2 film, Al2O3 film, Nb2O5 film, a laminated film prepared by laminating the TiO2 film on the Al2O3 film, a laminated film prepared by laminating the TiO2 film on the Nb2O5 film, or a low emissivity film be formed on a substrate and the SiO2 film be then formed on the undercoat film to serve as an SiO2 composite film. The undercoat layer has a function of imparting adhesion between the SiO2 film and the substrate, to improve durability of the film and to adjust the surface irregularity. By treating the composite film with water, it is possible to selectively form the silanol group.
It is also desirable that Al be mixed with Si material in advance when an SiO2 film is formed or 0.1xcx9c20% by weight, preferably 1xcx9c10% by weight of an Al component be doped relative to an Si component of the SiO2 film by the treatment after formation of the silanol group by water treatment. Since the Al component is doped, adsorption conditions of water to the silanol group are stabilized and as a result, the water-holding properties of the SiO2 film can be improved.
In this ease, it is desirable that another SiO2 film be formed between the undercoat layer and the substrate. It is also desirable that an SnO2 film be selected as a low emissivity film. If the SnO2 film formed through a laminated film prepared by laminating the SnO2 film and SiO2 film on the substrate in turn by a CVD method is selected, it is also possible to decrease interference of reflected color of a thin film and to prevent seepage of alkali (Na) from a glass substrate.
It is also effective to form an SiN film of a thickness of 1xcx9c20 nm as the undercoat layer for the purpose of preventing seepage of alkali (Na) from the glass substrate.
In the case where the substrate is composed of glass material, it is desirable that the substrate be burned at a temperature of 300xcx9c500xc2x0 C. after the water treatment to form silanol groups in an atmosphere in which 0xcx9c100% of nitrogen is mixed relative to air. With this burning, it is possible to remove organic substances adhering to the surface which becomes a repellant component and to improve the hydrophilic properties.
The thickness of the SiO2 film formed by the treating method of the present invention is adjusted to 1xcx9c100 nm, preferably 1xcx9c65 nm. The reason for this is that in the case of under 1 nm, it is not possible to sufficiently exhibit the hydrophilic properties and even though the thickness is over 100 nm, it is not possible to expect an increased effect.
A film with a thickness of this range formed by the sputtering method is very flat compared with the plasma CVD method or the vacuum deposition method, and the hydrophilic properties can be improved. The sputtering method also has a characteristic in that uniform hydrophilic film can be formed even on a substrate with a large area such as a mirror.